1. Field of the Invention
This invention relates generally to the field of rigid disc drive data storage devices, and more particularly, but not by way of limitation, to an improved structure for a rotary actuator and associated head-mounting mechanism.
2. Brief Description of the Prior Art
Disc drive data storage devices of the type known as "Winchester" disc drives are well known in the industry. In such machines, digital data are recorded on and retrieved from a thin layer of magnetizable material on the surface of spinning discs. The recording and retrieval of data--also referred to as "writing" and "reading", respectively--is accomplished using a transducer carried in a slider body which includes a self-acting hydrodynamic air bearing which "flies" the transducer a very small distance above the surface of the disc. This slider/transducer subassembly is sometimes referred to collectively as a head, and typically a single head is associated with each disc surface. The heads are selectively moved, under control of electronic circuitry, to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. In the current generation of rigid disc drive products, the most commonly used type of actuator is the rotary moving coil actuator, sometimes referred to as a rotary voice coil motor actuator.
In a typical disc drive incorporating a rotary voice coil motor actuator, a pivot shaft is fixedly mounted to the disc drive housing and an actuator body is rotatably coupled to the shaft by an arrangement of ball bearing assemblies. On one side of the rotatable actuator body is mounted a flat wound coil of conductive wire. This coil is held by the actuator body within the magnetic field of an array of permanent magnets, which are in turn also fixedly mounted to the disc drive housing. When controlled DC current is applied to the coil, a magnetic field is created about the coil which interacts with the magnetic field of the permanent magnets in accordance with the well known Lorentz relationship. This interaction is used to rotate the actuator body about the pivot shaft in a controlled manner.
Also mounted to the actuator body, typically on the side opposite the coil, is a vertically aligned array of heads. The heads are usually connected to the actuator body through an arrangement of rigid arms and a rather flexible head suspension or flexure. The flexure serves to apply a "downward" force on the head, i.e., toward the disc surface, to counterbalance the hydrodynamic lifting force of the slider, and maintain the head in a desired close spatial relationship to the spinning disc. The flexure is compliant in the roll and pitch axes of the head to allow the flying characteristics of the head to follow minor variations in the disc surface, and stiff in the yaw and in-plane axes of the head to permit accurate placement of the head relative to the data recorded on the disc.
Historically, the heads and flexures were manufactured as a subassembly, with the ends of the flexure opposite the heads including a relatively heavy mounting plate. This mounting plate was attached to a rigid arm using machine screws, and the arms were in turn then attached to the actuator body, again with machine screws. This type of assembly did, however, have the drawback of limiting the proximity of adjacent head arm assemblies due to the vertical height of the screw heads.
Another drawback to the type of actuator assembly described above relates to the materials used for each of the major components. Typically, the flexure assembly used to support the heads was fabricated from 300 series stainless steel for strength and flexibility. The actuator body itself was made from aluminum or magnesium to minimize the mass of the moving portion of the actuator, while the head arms were also typically of magnesium. The result of these various materials was differential thermal expansion, i.e., the various components expanding and contracting at different rates over the specified thermal range of the disc drive. Thus the problems of differential thermal expansion, along with the previously mentioned difficulties in reducing the spacing between adjacent discs and the additional costs associated with drilling and tapping screw holes and the insertion and torquing of the screws, lead to the development of the next generation of head mounting technology.
In order to mount the heads in closer proximity, and thus bring the discs closer together and increase the disc drive capacity, swage mounting of the heads to arms precast as integral parts of the actuator body was the next logical step. Since the swaging could be accomplished on all heads in a single operation, the arms could be made part of the actuator body, eliminating the manufacturing step of attaching the arms to the actuator body. This type of assembly also contributed greatly to the consistent vertical alignment of the heads in the array, which allowed greater accuracy in the alignment of the heads with the data tracks. It did, however, still suffer from problems of differential thermal expansion, due to the mixture of component materials.
With the advent of smaller and smaller disc drive form factors, attempts to lessen the spacing between heads and discs lead to a third generation of actuator assembly. This third generation structure combined the head/flexure assemblies with the head arms, with both of these major components made of stainless steel, to reduce differential thermal expansion problems. The head arms were formed with circular openings at the end opposite the flexure assembly.
The actuator body was, in turn, simplified to a cylindrical shape, with a projecting flange at one end and external threads on the other end. During manufacture, a plurality of head/flexure/arm assemblies was placed over the cylindrical actuator body, along with appropriate spacers, and brought to rest in contact with the protruding flange. A coil, which also included a circular mounting ring which acted as one of the spacers in the head stack, was similarly mounted to the actuator body extending away from the head arms. Once the desired configuration of heads and coil was complete, a threaded nut was screwed onto the external threads on the end of the actuator body opposite the flange, and tightened to squeeze the head/flexure/arm assemblies and coil together.
This third type of assembly did, however, also have a disadvantage. Disc drives of this type were frequently made in the 2.5" form factor for use in portable or laptop computer systems. Disc drives employed in such systems typically are required to be able to withstand non-operating mechanical shocks in excess of 100 G. Furthermore, when the disc drive experienced "runaway" conditions, the actuator assembly could contact the limit stop of the unit at high speed, subjecting the actuator to up to 1000 Gs. Future disc drives, having more powerful actuator motors, can be expected to provide even greater forces. This meant that the head stack had to be clamped together with sufficient force to prevent any shift in the relative positions of the heads, as well as the coil. In order to accomplish this large clamping force, the flange on the actuator body must be fairly robust, and the clamping force is controlled by the amount of torque applied to the locking nut at the end of the actuator body opposite the flange.
As previously mentioned, however, the actuator body was typically made of magnesium or other lightweight material, while the flexure/arm assemblies were made from stainless steel. Even if, as is the case of the preferred embodiment of the present invention, the actuator body were also to be made of steel, subtle differences between the coefficients of thermal expansion for various types of steel produced similar, if lesser, differential thermal expansion characteristics. This lead to certain problems with differential thermal expansion, in this case, in the axial direction of actuator body/ball bearing structure. Specifically, when the disc drive was subjected to thermal excursions within the specified allowable thermal range, the expansion and contraction of the mounting rings of the head arms and coil tended to increase and decrease the clamping force of the nut/flange combination. In order to compensate for this variation, the flange of the actuator body was dimensioned to provide a certain amount of compliance, so that the clamping force could be maintained within a desired range.
If the stack of actuator components were assembled without any compliance--that is, the flange on the actuator body was dimensioned to be substantially noncompliant and the amount of clamping force was controlled solely by the amount of torque applied to the nut during manufacture--inevitable variations in the manufacturing process, such as torquing tool calibration, operator induced variabilities, and component tolerance variations, resulted in unacceptable variation in the clamping force applied to the assembly. That is, for example, small variations in the torque applied to the nut caused relatively large and unacceptable variation in the clamping force applied.
Furthermore, vibration and thermal excursions which would be within the specified acceptable range for the finished disc drive unit could cause loosening of the nut in such non-compliant assemblies, and component "creep" could also result in unacceptable variation in the axial load force applied to the actuator assembly.
Thus the requirement that the actuator body flange be robust--or non-compliant--to provide sufficient clamping force, and at the same time compliant, to allow for differential thermal expansion, lead to unacceptable compromises in the actuator design.
Designing a compliant flange with an acceptable spring rate is difficult, and may well be impossible as requirements for greater and greater shock tolerance are made by the marketplace. That is, the spring rate must be as low as possible, so that the axial load changes little for changes in deflection of the flange. But the axial load must be as high as possible to prevent relative movement of the heads and coil. Since a low spring rate gives a low yield strength, a tradeoff must be made. Unfortunately, a flange that is strong enough to contribute to the required axial load force would give essentially no deflection capability within the head/coil stack to compensate for differential thermal expansion of the various components.
A need clearly exists, therefore, for a head stack design that provides both high axial clamping force and sufficient compliance to allow for differential thermal expansion and manufacturing and environmental considerations noted above.